This invention relates to an improved process for preparing calcined gypsum and more particularly pertains to the formation of low consistency calcium sulfate hemihydrate produced by atmospheric calcination processes, particularly continuous kettle operations.
Ordinarily calcined gypsum from good quality gypsum by atmospheric processes, known under various names, such as stucco, plaster of Paris, molding plaster, building plaster and the like, consists of the beta hemihydrate of calcium sulfate, CaSO.sub.4.1/2H.sub.2 O. This material is capable of being reconverted into calcium sulfate dihydrate, CaSO.sub.4.2H.sub.2 O, by mixing it with proper amounts of water. The theoretical water required to convert the stucco to gypsum dihydrate is only 18.7 parts by weight per 100 parts of pure hemihydrate CaSO.sub.4.1/2H.sub.2 O. However, in order to produce a workable aqueous slurry in a modern automated gypsum board plant the stucco will be mixed with amounts of water in excess of that required for hydration. Thus the mixing water required may vary from about 85-100 parts of water per 100 parts of the calcined gypsum by weight. The excess, about 67 to about 82 parts of water, present in the slurry will be removed by drying the board. Ordinarily, gypsum board dryers in automated gypsum board manufacturing lines will remove this water, for example, by maintaining a drying air temperature at about 400.degree. F. and requiring a drying time of about 40 minutes. Of course, the time-temperature relationship is variable from one processing plant to another primarily depending upon the particular gypsum source and processing equipment available.
Continuously calcined gypsum stucco, prepared in a kettle or a rotary calciner for example, processed in the usual manner comprises stucco particles with innumerable fissures and imperfections resulting from the violent dehydration process occuring under the harsh conditions of atmospheric calcination. These particles when added to the water and agitated, as during the slurry mixing process step, break up into smaller fragments thus exposing large surface areas of calcium sulfate, and therefore requiring a high amount of mixing water to obtain a fluid slurry. The additional water forms voids upon evaporation and thus greatly impairs the strength of products formed from this stucco.
It has become the custom in the gypsum industry to describe the amount of water, expressed in cubic centimeters or grams required to be added to 100 grams of calcined gypsum to produce a slurry with a standard fluidity such as the amount in a given time to flow through a standardized container as the "consistency" of that plaster. This is usually expressed by merely a number, it being understood, however, that the number means cubic centimeters or grams of water per 100 grams of that stucco. In addition, it has further become the custom in the gypsum industry to delineate particular consistencies for particular mixing conditions such as a "7 second consistency" wherein the stucco is dispersed by mechanical mixing in a laboratory mixer at a high sheer intensity and for a standard time of 7 seconds.
For many years various additives have been admixed with ordinary calcined gypsum for the purpose of producing a lower consistency than that which is characteristic of the particular calcined gypsum. Such materials may be exemplified by various ligno sulfonates and synthesized non-lignin based high molecular weight sulfonates as being a class of stucco dispersants having particular efficacy. These chemicals are considered chemical consistency reducers and apparently they act to reduce the interaction between the stucco fragments in aqueous suspension thus requiring less water to make a slurry of standard fluidity. Such segmented stuccos ordinarily have a consistency of around 68-79 cc.